Check should write warnings when slicing or allow don't match

[ahus1]

When writing checks it is easy to add typos to withSlicing() and allow().

It would be helpful for me

• if degraph would issue a warning when pattern (either anonymous or named) doesn't match any class
• if an allow references a name that doesn't exist

Thanks!

[ahus1]

Closing my own issue due to inactivity and my move to https://github.com/TNG/ArchUnit

[schauder]

It's still a valid issue though.